Stellar Evolution and the Herzsprung-Russell Diagram

Stellar Evolution and the
Hertzsprung-Russell Diagram
Based on a presentation by
Francis P. Wilkin
Department of Physics and Astronomy
Union College
7/15/13 at RPI
Dudley Observatory Astronomy Institute: Planetary Science
and Astronomy for the Next Generation of Science Standards
Outline of Presentation
• Stars: Their properties and how they work
• H-R diagram: Summarizes properties and displays evolution
By plotting Luminosity on vertical axis, temperature on horizontal
• Evolution: Why and how it occurs
A star is a massive, self-luminous ball of gas held together by its
self-gravity, and that shines, or used to shine, due to energy
released by nuclear reactions in its core.
Self-luminous: not reflecting light like a planet. Shines
because it is HOT (not because of nuclear reactions).
Nuclear bombs: not held together
by gravity –> not man-made stars!
Nuclear fusion reactions: convert light element nuclei to
heavier nuclei, also converting matter to energy.
Dead stars: white dwarfs, neutron stars
(black holes? How to tell?)
Not stars: Brown Dwarfs (mass from 13-80 x Jupiter)
How a Star Works
in Four Ideas
• Energy escapes from the photosphere
(surface) as light at the location where the
atmosphere becomes transparent
• Energy is liberated in the core by nuclear
fusion reactions (usually converting
Hydrogen to Helium: protons to alpha
particles). This fusion requires fuel, plus
high temperature and density
• Energy is “transported” from the core to
the photosphere by a combination of
photons (radiative transport) and boiling
fluid motion (convective transport)
• At all locations in the star, the pressure
force from hot gas opposes the weight
(due to gravity) of the mass above it
“hydrostatic” or “gravitational
equilibrium”.
The Hertzsprung-Russell (H-R) Diagram
Vertical axis: luminosity
(the amount of energy)
Horizontal axis:
surface temperature,
spectral type
“Sequences” are
types of stars
All main sequence stars
fuse H->He in core
Some combinations of L,T
Never occur! Why?
Stellar Evolution: The Life cycle of a Star
• Unlike biology, does not refer to
subsequent generations,
only the change in a given star with time
• Why must a star evolve?
• Isn’t it in equilibrium?
No! It continuously loses energy to space!
Core’s chemical composition
continuously changing
-> must evolve!
Also loses mass as a wind:
a key driver of evolution at the very end
Sun’s life:
Formation in an interstellar cloud (mostly H)
Protostar (accumulating more mass)
Pre-main Sequence star (internal adjustment)
Main Sequence star (current phase, longest; H->He in core)
Red Giant star (He->C fusion in core)
Planetary Nebula phase (sheds atmosphere)
White Dwarf (“dead”, cooling)
Stellar Properties: Brightness and Magnitudes
Magnitudes: backwards (logarithmic) scale used for brightness.
Negative numbers are brighter than positive! Each magnitude
corresponds to a factor of about 2.51
A difference of 5 magnitudes corresponds exactly to a factor of
100 in brightness
Apparent “visual” magnitudes: How bright a star appears to us on
Earth.
Venus: m = -4.5,
Vega: m = 0.0,
Polaris: m = 2.0
Faintest star visible to naked eye: m = 6
Polaris is about 2.5 x 2.5 = 6.3 times fainter than Vega!
Absolute magnitude: the brightness an object would have if at the
standard distance of 10 parsecs (about 32 LY). This is a star’s true
brightness.
Brightness, Distance, and Luminosity I.
Looking at the sky, we can’t tell whether a bright star is closer or
farther than a faint star
Vega (magnitude 0.0) is brighter than Deneb (mag 1.25)
Vega: d = 25 light years (LY),
luminosity 37 Lsun
Deneb: d = 1400 LY,
luminosity 54000 Lsun
Deneb is more luminous than Vega!
Brightness, Distance, and Luminosity II.
• Luminosity: the total amount of energy emitted by a star per
unit time
• Flux (or brightness): the amount of energy crossing a unit area
per unit time (normal incidence)
4 π d2 F = L
Surface Temperature and Color
• A star is essentially opaque and its spectrum strongly resembles the theoretical
“Blackbody” thermal spectrum, depending only on the temperature.
Albireo (double star)
Note: a blue star is HOTTER than a red one, despite the frequent use of red for
hot, and blue for cold in false-color images!
Stellar Spectral type goes from hot to cool in the sequence O,B,A,F,G,K,M
“Oh, be a fine girl (guy), kiss me!”
Stellar Sizes: The Stellar Zoo
http://www.youtube.com/watch?v=HEheh1BH34Q
Stellar Size related to Temperature and Luminosity
• An opaque thermal source (blackbody) emits
amount of energy σT4 for every square meter of
surface.
• A sphere has surface area 4 π R2.
(σ is the Stefan-Boltzmann constant)
• Thus, the star’s luminosity L = 4 π R2 σ T4
Solve for R, the radius of a star:
R = (L/4 π σ)1/2/T2
H-R Diagram (Size Matters)
Key Stellar Properties
Sun: Mass=2x1033 kg (about 1000 Jupiters!),
Luminosity=4x1026 W, Radius=7x108 m (about 100 Earths!),
Teff=5800 K,
Tcore=1.6x107 K
Masses: 0.08 – 150 Msun
Luminosity 10-4-106 Lsun
Radius: 10-2 – 103 Rsun
Temperature 3000K – 50,000 K (exceptions)
Huge range of size, luminosity
Smaller ranges in temperature, mass
Star Formation
A,B) Stars form by gravitational collapse in interstellar hydrogen clouds
C) Rotation causes a flattening, and a disk forms around the protostar,
which is still accumulating mass and invisible at optical wavelengths (Not in H-R diagram)
D) Planets form in the disk, nuclear reactions occur in the pre-main sequence (T Tauri)
phase
Main Sequence Stars
•
•
•
90% of all stars
Defining characteristic:
H-> He fusion in core
Luminosity roughly proportional
to M3
•
Long-lasting: star consumes
inner 10% of hydrogen.
(Compare luminosity to
fuel supply to find lifetime)
tMS proportional to 1/M2
General trends:
• More massive stars are
larger, hotter, more luminous
• Roughly diagonal from
upper left (luminous, hot)
to lower right (faint, red)
Post-Main Sequence
Evolution
• Core runs out of fuel and must
contract; core temp rises
• H->He continues in a shell
surrounding the core
• Atmosphere expands and cools
• Details depend sensitively on
total mass: consider “low mass”
stars like the sun, and “high
mass” stars over 8 Msun.
• “Very low mass” stars end their
lives after the main sequence
with no further nuclear fuels.
• Post-M-S evolution may last
1/10 as long as M-S did
• (Sun: 1 billion vs. 10 billion)
Low-Mass Star Death
Planetary Nebula Ejection
Not an explosion
but a strong wind
30-70% of mass
may be ejected
Lasts 10-30 thousand years
Central star becomes white dwarf
White Dwarfs are Forever!
• New type of pressure opposes gravity: Quantum-mechanical degeneracy
pressure.
• Electrons repel each other when highly compressed (not due to charge)
• Requires no energy source – keeps working as the star cools
• Star maintains constant radius
(about same as Earth)
• Example: Sirius B
• Maximum mass 1.4 Msun or collapse!
Incredible density!
0.6 Msun in one Earth volume!
Nearly 106 g/cm3 !
Death of Massive Stars
• Concentric shells of successively more
massive fuel nuclei
• The ashes of each reaction are the fuel for
the next inner shell.
• Each reaction is less efficient than the
previous: they buy very little time
• Iron accumulates because energy cannot be
gained by making heavier elements (it has
the most stable nucleus)
• The iron core is supported by degeneracy
pressure, just like a white dwarf
• When the core of Iron reaches 1.4 Msun, it
collapses, triggering a supernova!
Massive Star Supernova
(“Type II”)
• Core mass exceeds 1.4 Msun
and collapses
• Electrons collide with protons to make
neutrons (removing the pressure source!)
• Collapse of center until reaching
nuclear density, size about 10 km
• Outer parts fall onto “neutron core”
and bounce
• Neutrinos provide additional push
• Convection and non-sphericity may be critically important
• Envelope flies out as supernova remnant
• Core collapses into either a black hole or neutron star
• Ejected matter provides the heavy elements the Earth, and our
bodies, are made of
Pulsars: Rapidly-Spinning, Magnetized Neutron Stars
• Mass roughly 1.35-2 Msun (limits uncertain)
• Size about 12 km radius
• Gravity is opposed by neutron degeneracy pressure (requires
no energy source, lasts forever)
• Lighthouse model: beams
along magnetic axis
Binary Stars: Mass Transfer
Mass transfer slows the evolution of the losing star, and
speeds the evolution of the gainer.
It can push a white dwarf over the maximum mass limit,
causing a type Ia supernova explosion!
Mass striking a surrounding disk can glow in x-rays!
Many possibilities
Conclusions
• Stellar evolution is primarily determined by the
initial stellar mass (higher mass, shorter life)
• Nuclear fusion reactions provide the energy source
for the stars
• Low mass stars end up being white dwarfs
• High mass stars explode as supernovae, leaving
either a neutron star or a black hole
• Many details of star formation, and supernova
explosions, are still unknown
• All the elements beyond the first five (H,He,Li,Be,B)
were formed in stars : we are stardust